Trace metals and nutrients in aerosols over the tropical and subtropical North Atlantic Ocean

Abstract

In the first part of this thesis an overview is given of methods available for the analysis of nanomolar nitrate and phosphate in seawater before going on to describe in more detail a system built in our laboratory comprising liquid waveguide capillary cells connected to a conventional segmented-flow autoanalyser. This approach is suitable for routine field measurements of nitrate and phosphate and achieves detection limits of < 1 nM phosphate and nitrate. Investigations were conducted into interferences of silicate and arsenate with the analysis of nanomolar concentrations of SRP, the effect of sample filtration on the measurement of nanomolar nitrate + nitrite and SRP concentrations, and the stability of samples during storage are described. Arsenate interference scaled linearly with phosphate concentrations of up to 50 nM, resulting in an overestimation of SRP concentrations of 4.6 ± 1.4% for an assumed arsenate concentration of 20 nM. The interference effect of added Si(OH)4 on the measured SRP signal is small at the dissolved silicon concentrations typically found in oligotrophic waters. Filtration of surface seawater samples resulted in a decrease in concentration of 1.7 – 2.7 nM (±0.5 nM) SRP, and a small decrease in nitrate concentrations which was within the precision of the method (±0.6 nM). A stability study indicated that storage of very low concentration nutrient samples in the dark at 4°C for less than 24 h resulted in no statistically significant changes in nutrient concentrations.
The second half of this thesis presents a dataset from aerosols collected at the Cape Verde Atmospheric Observatory (CVAO) between July 2007 and July 2008 and collected during a research cruise in the (sub-) tropical North Atlantic Ocean in January 2008. Total acid digestion followed by ICP-MS analysis reveals that the total elemental composition of the dust is close to average crustal composition and shows a high degree of consistency. Based on elemental composition data alone, dust collected on the cruise appears similar to dust collected at the CVAO. Zn and Pb are elevated above crustal values indicating an anthropogenic source, but show an association with periods of high mineral dust concentration. Ultrapure water leaches of dust samples combined with analysis for nutrients and trace metals show a picture of atmospheric concentrations of soluble trace metals and nutrients throughout the year in the study region. Estimated dry deposition fluxes for Fe, and inorganic N and P show a marked difference between summer and winter, with higher Fe and P deposition during winter when mineral dust concentrations are at their greatest, while N inputs are more constant throughout the year. Relative to Redfield ratios, atmospheric inputs are greatly enriched in Fe relative to N and P.

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